Mesh connected brake array for electrical rotating machines

ABSTRACT

In the present invention, several polyphase devices are connected together: an inverter ( 420 ), and electrical rotating machine ( 440 ), and a resistive load or braking resistor ( 430 ). The purpose of the resistive load is to dissipate excess electrical power which may be produced when the inverter acts to slow down the rotating machine ( 440 ), causing the rotating machine to act as a generator. In common art, this resistive load is a single DC resistor coupled to the DC link of the inverter via a separate resistor control transistor. In the present invention, the resistive load is a mesh connected array of resistors, and is electrically connected to the same inverter output terminals that the rotating machine is connected to. When it is desired that the resistors absorb energy, for example from a braking operation, then the harmonic content of the inverter output is adjusted, thus placing voltage differences across the resistor array ( 430 ) and causing current to flow in the resistors.

TECHNICAL FIELD

The present invention relates to high phase order electrical rotatingmachines.

BACKGROUND ART

When the drive motor is used as a brake during operation of an elevatorwith the induction motor driven by an inverter, the rotating speed ofthe motor is higher than the frequency of the inverter, and regeneratedpower is formed in the motor. As this regenerated power flows into theDC circuit of the inverter, a resistor in the DC circuit absorbs theregenerated power. A number of approaches for controlling this have beendisclosed; for example Kitaoka and Watanabe (U.S. Pat. No. 4,678,063)disclose an elevator control system connected to a source of three-phasealternating current which is rectified by a converter to direct currentwhich is converted to a variable-voltage variable-frequency A.C. voltagewhich, in turn, drives the elevator hoist motor. A resistor and a switchare connected across the D.C. terminals of the converter. When the motoris operating in the regenerative mode, the switch is closed to permitthe regenerated circuit to flow through the resistor, which dissipatesor consumes the regenerated power. When the regenerated power beingconsumed by the resistor is detected to exceed a predetermined value,the excess regenerated power is returned to the A.C. source through aregenerative inverter. Kanzaki and Yamada (U.S. Pat. No. 5,420,491)disclose the conversion of an analog DC link bus voltage in an inductionmotor drive to a digital DC link bus voltage so that if the DC link busvoltage exceeds an ON voltage threshold of a switch and this conditionexists a holding time later, the switch closes, thereby allowingregenerated power in the DC link to be dissipated in a resistorconnected across the DC link. And if the DC link bus voltage falls belowan OFF voltage threshold and that condition exists a latching timelater, the switch is opened so that no regenerated power may bedissipated through the resistor. Systems disclosed in the prior art usea variety of methods for sensing when regenerated power on the DC supplyline, but they all use a switch (a transistor in the case of Kitaoka andWatanabe) to allow the regenerated power to flow through the resistor.

DISCLOSURE OF INVENTION

In general terms the present invention provides a motor control systemin which power regenerated during braking is dissipated or conserved byaltering the composition of the drive waveform. This approach to doesnot require additional switches beyond those of the inverter outputstage to control the flow of regenerated power through the resistor, asis the case with prior art systems.

The invention is an apparatus for dissipating regenerated powergenerated from an induction motor having more than three phases, andcomprises an inverter providing more than three different phases of analternating current output drive waveform, a regenerated power sink, acontroller for altering the harmonic content of the output drivewaveform provided by the inverter, and a detector for detectingregenerated power when the motor is used as a brake. The inverter isconnected to the motor with a first mesh connection, in which each motorphase is electrically connected to a first inverter terminal and asecond inverter terminal L inverter terminals distant from the firstinverter terminal in order of electrical phase angle, where L is theskip number, and the phase angle difference between the pair of inverterterminals to which each motor phase is connected is identical for eachmotor phase. When the detector detects regenerated power, it signals thecontroller, which alters the harmonic content of the output drivewaveform so that it contains one or more additional harmonic components.The regenerated power sink has at least three elements and is connectedto the inverter with a second mesh connection in which each element iselectrically connected to a first inverter terminal and a secondinverter terminal, and a phase difference across each element is zero inthe absence of the additional harmonic component, and is not zero in thepresence of the additional harmonic component. This means that when themotor is used for braking, the regenerated power is diverted through thesink; however, under normal powered operation, no power is divertedthrough the sink.

In one embodiment, where the motor is a high phase order motor,resistors are connected to the inverter output such that there is noelectrical phase angle difference across the ends of the resistors, andtherefore no voltage is developed across the resistors, and no currentflows through them. This is achieved by a combination of the connectiongeometry and the composition of the drive waveform. The resistor arraytherefore does not dissipate any power under these running conditions,when the motor is operating in a driven state. Under braking conditions,the motor regenerates to the DC power supply to the inverter, leading toan increase in the DC supply voltage to the inverter. The increase isdetected, and the composition of the drive waveform is modified so thatthere is now an electrical phase angle difference across the ends of theresistors, and ac power now flows through the resistor array, thusdissipating the brake current.

In the present invention, several polyphase devices are connectedtogether: an inverter, and electrical rotating machine, and a resistiveload or braking resistor. The purpose of the resistive load is todissipate excess electrical power, which may be produced when theinverter acts to slow down the rotating machine, causing the rotatingmachine to act as a generator. In common art, this resistive load is asingle DC resistor coupled to the DC link of the inverter via a separateresistor control transistor. In the present invention, the resistiveload is a mesh-connected array of resistors, and is electricallyconnected to the same inverter output terminals that the rotatingmachine is connected to.

As I have previously disclosed, the impedance of a mesh connectedpolyphase load may be adjusted either by changing the spanning value ofthe mesh connection, or by changing the harmonic order produced by theinverter. In the present invention, the rotating machine and the meshconnected resistor array are internally connected with different meshconnections; optionally the rotating machine may be star connected. Themesh connection of the resistor array is selected so that when therotating machine is operated normally at its design point, no voltagedifference is present across any of the resistors. Essentially, a meshconnection and harmonic order are selected such that the impedance ofthe resistor array is infinite.

When it is desired that the resistors absorb energy, for example from abraking operation, then the harmonic content of the inverter output isadjusted, thus placing voltage differences across the resistor array andcausing current to flow in the resistors.

BRIEF DESCRIPTION OF DRAWINGS

For a more complete explanation of the present invention and thetechnical advantages thereof, reference is now made to the followingdescription and the accompanying drawings, in which:

FIG. 1 illustrates how the winding terminals of a polyphase motor may beconnected to a polyphase inverter.

FIG. 2 illustrates a plurality of ways in which the polyphase invertermay be connected to a polyphase motor.

FIG. 3 illustrates how winding terminals of a motor connected to apolyphase inverter in a particular fashion may be driven by the inverterwith various phase angles.

FIG. 4 is a block diagram showing regenerative braking.

FIG. 5 illustrates the connection of a polyphase inverter to a polyphasemotor and a resistor array.

BEST MODE FOR CARRYING OUT THE INVENTION

In an induction machine, each phase winding set can be described by twoterminals. There may be a larger number of terminals, but these arealways grouped in series or parallel groups, and the entire set can becharacterized by two terminals. In a star connected machine, one ofthese terminals is driven by the inverter or power supply, while theother terminal is connected to the machine neutral point. All currentflows through one terminal, through the neutral point into otherwindings, and though the driven terminals of the other phases. In amesh-connected machine, these two terminals are connected directly totwo different supply points. An example of how this may be done is shownin FIG. 1, in which the stator slots 4 are shown as straight linesrunning down the inside of the stator, and inverter terminals 2, areshown as circles, alongside which is marked phase angles of each of theinverter terminals. Electrical connections 3 between the windingterminals in stator slots 4 and inverter terminals 2 are represented bydashed lines. Two winding halves are displayed opposite one another, andare actually joined to one another, although this is not shown. Theconfiguration describes a 9 phase machine connected with an L=4connection—identical to FIG. 2 e.

In contrast to three phase systems, in which there are only threeinverter terminals and six motor windings terminals, in a high phasecount system with N phases, there are N inverter terminals and 2N motorwindings terminals. There are thus a substantial number of choices forhow an N phase system may be mesh connected. This set of choices isgreatly reduced by rotational symmetry requirements, specifically eachwinding must be connected to two inverter terminals with the sameelectrical angle difference between them as for every other winding.

A simple graphical schematic of the permissible inverter to motorwindings connections may thus be described, for a polyphase motor havingN phases. FIG. 2 shows N evenly spaced points and a center point. Eachof these points represents an inverter terminal 2, to which one of theterminals of each of one or more motor windings 1 may be connected.Permissible connections of the N phase windings are either from thecenter point, to each of the N points on the circle (this being the starconnection shown as FIG. 2 a) or from each of the N points to anotherpoint L points distant in the clockwise direction, where L is onegreater than the number of skipped points (inverter terminals). It willbe noted that for each L from 1 to (N−1)/2 there is a corresponding Lfrom N/2-1/2 to N that produces a mirror image connection.

FIG. 2 shows all permissible connections for a 9 phase system from L=1to L=4 as well as the star connection. Noted on the star connectiondiagram are the relative phase angles of the inverter phases drivingeach terminal. For a given inverter output voltage, measured between anoutput terminal and the neutral point, each of these possibleconnections will place a different voltage on the connected windings.For the star connection, the voltage across the connected windings isexactly equal to the inverter output voltage. However, for each of theother connections, the voltage across a winding is given by the vectordifference in voltage of the two inverter output terminals to which thewinding is connected. When this phase difference is large, then thevoltage across the winding will be large, and when this phase differenceis small, then the voltage across the winding will be small. It shouldbe noted that the inverter output voltage stays exactly the same in allthese cases, just that the voltage difference across a given windingwill change with different connection spans. The equation for thevoltage across a winding is given by: 2*sin((phasediff)/2)*Vout wherephasediff is the phase angle difference of the inverter output terminalsdriving the winding, and V is the output to neutral voltage of theinverter.

Thus, referring to FIG. 2, when L=1, the phase angle difference is 40degrees, and the voltage across a winding is 0.684Vout. When L=2, thephase angle difference is 80 degrees, and the voltage across the windingis 1.29Vout. When L=3, the phase angle difference is 120 degrees, andthe voltage across the winding is 1.73Vout. Finally, when L=4, the phaseangle difference is 160 degrees, and the voltage across the winding is1.97Vout. For the same inverter output voltage, different connectionsplace different voltage across the windings, and will cause differentcurrents to flow in the windings. The different mesh connections causethe motor to present different impedance to the inverter. In otherwords, the different mesh connections allow the motor to use the powersupplied by the inverter in different rations of voltage and current,some ratios being beneficial to maximize the torque output (at theexpense of available speed), and some ratios to maximize the speedoutput (at the expense of maximum available torque).

To deliver the same power to the motor, the same voltage would have tobe placed across the windings, and the same current would flow throughthe windings. However, for the L=1 connection, to place the same voltageacross the windings, the inverter output voltage would need to be muchgreater than with the L=4 connection. If the inverter is operating witha higher output voltage, then to deliver the same output power it willalso operate at a lower output current. This means that the L=1connection is a relatively higher voltage and lower current connection,whereas the L=4 connection is a relatively lower voltage, higher currentconnection.

The L=2 connection is desirable for low speed operation, where itincreases the overload capabilities of the drive, and permits muchhigher current to flow in the motor windings than flow out of theinverter terminals. The L=4 connection is desirable for high speedoperation, and permits a much higher voltage to be placed across thewindings than the inverter phase to neutral voltage. This change inconnection is quite analogous to the change between star and deltaconnection for a three phase machine, and may be accomplished withcontactor apparatus. However the number of terminals renders the use ofcontactors to change machine connectivity essentially impracticable.

There is, however, an additional approach available with high phaseorder inverter driven systems.

The inverter, in addition to being an arbitrary voltage and currentsource, is also a source of arbitrary phase AC power, and this outputphase is electronically adjustable. Any periodic waveform, including analternating current may be described in terms of amplitude, frequency,and phase; phase is a measure of the displacement in time of a waveform.In a polyphase inverter system, phase is measured as a relative phasedisplacement between the various outputs, and between any pair ofinverter terminals, an electrical phase angle may be determined. In thecase of conventional three phase systems, this electrical phase angle isfixed at 120 degrees. However in polyphase systems this phase angle isnot fixed. Thus, while the machine terminals 1 . . . 9 may be fixed intheir connection to inverter terminals 1 . . . 9, the phase relation ofthe inverter terminals connected to any given motor winding terminals isnot fixed. By changing the inverter phase relation, the impedance thatthe motor presents to the inverter may be changed. This may be donewithout contactors.

Fundamental phase relation is both the relative electrical angle of eachwinding terminal and relative phase relation of the currents drivingeach winding terminal, such that the stator develops the lowest polecount without discontinuities. In a two pole machine, driving withfundamental phase relation causes the electrical angle of each windingterminal, as well as the phase angle of the currents driving eachwinding terminal, to be equal to the physical angle of the winding slotassociated with that winding terminal. In a four pole machine, the phaseangle is equal to double the physical angle of the slot, and in generalfor an N pole machine the electrical angle between any two slots, andthe electrical phase relation of the currents driving those two slots,is equal to N/2 times the physical angle between those two slots.

With reference to FIG. 3, a 9 phase machine is connected to the invertersystem using the L=4 mesh. One terminal of each of two windings 1 isconnected to each inverter terminal 2. When driven with ‘first order’phase differences, then the results are as described above for the L=4mesh. However, if the phase angles are adjusted by multiplying eachabsolute phase reference by a factor of three, then the phasedifferences placed across each winding become the same as those found inthe L=3 case, although the topological connectivity is different. If thephase angles are adjusted by a multiplicative factor of five, then thevoltages across windings become like those of the L=2 case, and with amultiplicative factor of seven, the voltages become like those of theL=1 case. A multiplicative factor of nine causes all phases to have thesame phase angle, and places no voltage difference across the winding.

These changes in phase angle are precisely the changes in phase angleused to change the operating pole count of a high phase order inductionmachine, as described in others of my patent applications and issuedpatents.

If a high phase count concentrated winding induction machine is operatedby an inverter, but is connected using a mesh connection, then changesin pole count of the machine will be associated with changes in machineeffective connectivity. These changes in effective connectivity permithigh current overload operation at low speed, while maintaining highspeed capability, without the need for contactors or actual machineconnection changes.

Of particular value are machines connected such that the fundamental, orlowest pole count, operation is associated with a relative phase angleacross any given winding of nearly, but not exactly, 120 degrees. Inthese cases, altering the output of the inverter by changing theabsolute phase angles by a multiplicative factor of three, which mayalso be described as operation with the third harmonic will result inthe relative phase angle across any given winding becoming very small,and causing large winding currents to flow with low inverter currents. Aparticular example would be a 34 slot, 17 phase machine, wound with fullspan, concentrated windings, to produce a two pole rotating field. Thewinding terminations are connected to the inverter using the L=6 mesh.The relative phase angle of the inverter outputs placed across any givenwinding would be 127 degrees, and the voltage placed across this windingrelative to the inverter output voltage is 1.79 times the inverteroutput voltage. If the machine is then operated with a third harmonicwaveform, it will operate as a six pole machine. The relative phaseangle across any given winding is now 127*3mod 360=21 degrees, and thevoltage placed across the winding relative to the inverter outputvoltage is 0.37 times the inverter output voltage. Simply by changingthe inverter drive angles, the Volts/Hertz relationship of the motor isincreased, and inverter limited overload capability is enhanced.

To determine the ideal L, the number of skipped inverter terminalsbetween the winding terminals of each phase of the motor, which wouldresult in the greatest change of impedance when the inverter drives themotor with substantial third harmonic, one would use the formula(N/3)−1, rounded to the nearest integer, for values of N (number ofphases in motor) not divisible by 3. When N is divisible by 3, one woulduse the formula N/3 to determine the skip number.

Other connectivity is certainly possible. The connection described abovewill tend to maximize machine impedance for the third harmonic, but willactually decrease machine impedance for fifth harmonic. A connectionthat most closely approximates full bridge connection, e.g. the L=8connection for the 17 phase machine described above, will show graduallyincreasing machine impedance for the 3^(rd), 5^(th), 7^(th), 9^(th),11^(th), 13^(th), and 15^(th) harmonics. This may be of particularbenefit, for example, with machines operated with square wave drive.Operation with high pole counts is not generally considered preferable,however it may be of benefit in the particularly desirable case ofoperating at high overload and low speed. The number of slots is notrestricted, nor are the number of phases or poles. In order to determinethe value of L in the winding to inverter connections, one may use theformula (N−1)/2, when N (number of motor phases) is an odd number. WhenN is even by may be divided into subsets of odd phase counts, theformula may similarly be used for the odd subsets.

Referring to now to FIG. 4, a converter 410 converts the alternatingcurrent 405 from a three-phase AC mains supply into a direct current. Aninverter 420 is connected to the DC side of the converter, and convertsthe constant DC voltage into a variable voltage, variable frequency(VVVF) AC voltage by pulse-width control. A motor 440 is connected tothe inverter. A regenerated power-detecting means in the form of avoltage detector 460 is adapted to detect the DC voltage supplied to theinverter. This is actuated when the voltage exceeds a predeterminedvalue such as to cause a controller 450 to change the composition of thedrive waveform produced by the inverter; regenerated power is therebydiverted to a regenerated power sink 430, which is also connected to theinverter. The regenerated power sink allows power regenerated by themotor to be efficiently and safely handled. The sink may be any devicethat dissipates or conserves power, and may be, by way of example, aresistor array, a battery array, a potential energy storage unit, or ameans of returning power to the AC mains supply.

In a preferred embodiment, inverter 420 is a 15-phase inverter and motor440 is a 15-phase motor. Referring now to FIG. 5, which shows 15 evenlyspaced points and a center point. Each of these points represents aninverter terminal 2, to which one of the terminals of each of one ormore motor windings 1 may be connected. FIG. 5 shows motor windingsconnected through an L=6 connection, and the motor is driven using athird harmonic drive waveform. This means that the phase angle is144×3=72 (432) degrees, and the voltage across the windings is 1.18Vout.FIG. 5 also shows resistors 6 which are connected to the inverterthrough an L=5 connection, giving a phase angle of 120×3=0 (360)degrees, or 0 (360) degrees effective phase angle using the thirdharmonic drive waveform. The voltage across the resistors under theseconditions is zero.

Under braking, the motor regenerates to the DC side of the inverter,leading to an increase in the DC voltage. This increase is detected, andthe controller changes the composition of the drive waveform.

For example, the fundamental content of the drive waveform may beincreased, which changes the effective phase angle to the windings from72 to 144 degrees, and the voltage across the windings increases to1.90Vout. Similarly the effective phase angle across the resistor matrixchanges from 0 to 120 degrees, and the voltage across the resistorsincreases to 1.73Vout.

It is important to note that the motor must always have current goingthrough it, as the motor cannot regenerate unless it is also activated.This means that harmonics may be added to the inverter output so thatthe resistor array is also activated. Alternatively the output can beswitched to a harmonic that both the resistor array and the motor cantolerate e.g. fundamental.

By adjusting the relative proportions of fundamental or fifth harmoniccontent of the drive waveform, the amount of power dissipated throughthe resistor array may be controlled to match the power regenerated bythe motor.

In the above description, the resistor array has the same number ofphases as the motor. The present invention also envisages a resistorarray that has fewer phases than the motor, but which may still beactivated in the manner disclosed above. For example, in the exampleabove, if the resistor array is a three phase array, and is connected ina delta configuration, then the angle between the phases is still 0/360degrees when driven by third harmonic.

The present invention has been described with regard to rotary inductionmotors, however it may be implemented with linear induction motors too,using similar techniques for changing winding impedance. Where thewindings of a linear or also of a rotary induction motor comprise singleinductors instead of coils, then inverter output phase angle may bealtered by an even multiplicative factor in order to effect impedancechanges. In some cases, the inverter may even multiply each phase angleby a fractional factor to vary the impedance of the motor.

The word “terminal” has been used in this specification to include anyelectrically connected points in the system—this may be a screw, forexample, or any electrical equivalent, for example, it may simplycomprise a wire connecting two components in a circuit.

In a similar sense, inverter output elements are commonly half bridges,but they may alternatively comprise other switching elements. Oneembodiment of the present specification has described two windingterminals connected to a single inverter terminal. The single inverterterminal referred to is intended to also include electrical equivalents,such as a device made of two inverter terminals that are electricallyconnected together.

INDUSTRIAL APPLICABILITY

The present invention has a number of applications where breakingoccurs, such as traction applications, including cars, trains, trams,and other winding applications, including cranes, excavators and thelike. While this invention has been described with reference to numerousembodiments, it is to be understood that this description is notintended to be construed in a limiting sense. Various modifications andcombinations of the illustrative embodiments will be apparent to personsskilled in the art upon reference to this description. It is to befurther understood, therefore, that numerous changes in the details ofthe embodiments of the present invention and additional embodiments ofthe present invention will be apparent to, and may be made by, personsof ordinary skill in the art having reference to this description. It iscontemplated that all such changes and additional embodiments are withinthe spirit and true scope of the invention as claimed below.

All publications and patent applications mentioned in this specificationare indicative of the level of skill of those skilled in the art towhich this invention pertains. All publications and patent applicationsare herein incorporated by reference to the same extent as if eachindividual publication or patent application was specifically andindividually indicated to be incorporated by reference.

1. Apparatus for dissipating regenerated power generated from aninduction motor having more than three phases, comprising: a) aninverter for the synthesis of N different phases of alternating currentoutput drive waveform, where N is greater than three, and connected tosaid motor with a first mesh connection, said first mesh characterizedin that: each motor phase is electrically connected to a first inverterterminal and a second inverter terminal L inverter terminals distantfrom the first inverter terminal in order of electrical phase angle,where L is the skip number, and the phase angle difference between thepair of inverter terminals to which each motor phase is connected isidentical for each motor phase; b) a regenerated power sink having atleast three elements and connected to said inverter with a second meshconnection; c) a controller for altering the harmonic content of saidoutput drive waveform; d) a detector for detecting said regeneratedpower, and for signaling to said controller upon such detection, whereinsaid controller causes the output drive waveform to comprise one or moreadditional harmonic components; and wherein said second mesh ischaracterized in that: each element is electrically connected to a firstinverter terminal and a second inverter terminal, and a phase differenceacross each element is zero in the absence of said additional harmoniccomponents, and is not zero in the presence of said additional harmoniccomponents.
 2. The apparatus of claim 1 in which said regenerated powersink is selected from the group consisting of: a resistor array, abattery array, a potential energy storage unit, or a means of returningpower to an AC mains supply.
 3. The apparatus of claim 1 wherein N is amultiple of three; wherein said regenerated power sink comprises threeelements; wherein said second mesh connection comprises a deltaconnection; and a phase difference across each element is zero when saidoutput drive waveform consists of third harmonic, and is not zero in thepresence of said additional harmonic components.
 4. The apparatus ofclaim 1 wherein said motor has 15 phases; wherein N is 15; wherein saidoutput drive waveform consists of third harmonic; wherein L is 6;wherein said second mesh connection characterized in that: each motorphase is electrically connected to a first inverter terminal and asecond inverter terminal 5 inverter terminals distant from the firstinverter terminal in order of electrical phase angle; and wherein saidcontroller causes the output drive waveform to comprise fundamental.